Electron beam discharge tubes



March 18, 1969 M- R. BENNETT 3,434,006

ELECTRON BEAM DISCHARGE TUBES Filed May 19. 1965 mm M M BY ddamaa United States Patent Ofice 3,434,006 ELECTRON BEAM DISCHARGE TUBES Maurice Ronald Bennett, Rayleigh, England, assignor to English Electric Valve Company Limited, London, England, a British company Filed May 19, 1965, Ser. No. 456,984 Claims priority, application Great Britain, May 22, 1964,

21,214/ 64 US. Cl. 315-30 5 Claims Int. Cl. H013 29/74 ABSTRACT OF THE DISCLOSURE This invention relates to electron beam discharge tubes and more specifically to electron beam discharge tubes of the kind in which the beam intensity is modulated by deflecting the beam with respect to an apertured or interrupted electrode which is interposed in its path so that the proportion of the cross-section of the beam which is intercepted by the electrode can be variedcommonly between zero interception and total interceptionby varying the deflection, For brevity tubes of this kind will hereinafter be termed deflection modulated beam tubes." Forms of deflection modulated beam tubes include inter alia storage tubes and cathode ray oscilloscope tubes. In the most usual type of arrangement the beam intercepting electrode is an apertured plate which is transverse to the undeflected direction of the beam and has an aperture so positioned and of such size that the beam passes through it, unobstructed, when no deflection is applied. Deflection is applied electrostatically by means of a pair of plates between which the beam passes on its way to the aperture, these plates being of course between the cathode or gun of the tube and the apertured plate. Obviously the application of deflection will cause the direction of the beam, after it has passed the deflection plates, to be at an angle to the direction of that portion of the beam which is on the cathode side of the deflection plates and, to impact on the final beam receiving surface (e.g. a fluorescent screen) at some point which is shifted with respect to the point at which it would have impacted had it not been deflected. Such shifting of the point of impact is usually most undesirable and it is therefore common practice to provide one or more additional copensating pairs of plates between the beam after first being deflected and which serve to compensate for and thus prevent shifting of the point of impact due to deflection for modulating purposes. The compensating pair or pairs of plates may be on the cathode side of the apertured plate or on the other side thereof or there may be a pair of compensating plates on each side thereof. The plates of the different pairsdeflecting plates and compensating platesare interconnected, usually inside the tube envelope, in the manner necessary to ensure that, when the same voltage is applied between the plates of the different pairs, each successive pair produces the direction of deflection required from it, so that the beam will be modulated as required without its final point of impact being shifted as a concomitant of modulation. Because the deflecting eifect of a given pair of plates with a 3,434,006 Patented Mar. 18, 1969 given voltage applied between them depends upon the distance, measured along the beam, of the said pair from the cathode or gun, it is necessary, if the beam which has passed all the pairs is to be unshifted as to its point of final impact, for the different pairs of plates in the known arrangement just described to differ in plate length or plate separation or both. This is a serious practical defect. Not only is it to some extent an expensive matter in itself to adopt different designs and/or inter-plate separations for the different pairs of plates but it is necessary also to satisfy close tolerances in manufacturing and mounting the different pairs of plates, for, because the same potentials are applied between all the pairs of plates, any failure either byreason of inaccurate manufacture or subsequent movement of individual plates inside the tube envelope, to satisfy the original design dimensions, is liable to cause incorrect compensation. This requirement of close tolerances is quite expensive to satisfy. The present invention seeks to avoid this defect and, in its preferred forms, uses pairs of plates which are all alike.

According to this invention a deflection modulated beam tube includes a pair of deflecting plates for deflecting the beam out of its original path for modulation, at least one pair of compensating plates for compensating for undesired shifting of the point of final beam impact, and separate connections brought outside the tube envelope from at least one electrode of each pair.

Preferably all the plates are alike and all the pairs have the same inter-plate separation.

As in known tubes the pair or pairs of compensating plates may include a pair on the cathode or gun side of the apertured or interrupted electrode across which the beam is deflected for modulating purposes and/or a pair on the other side of said electrode.

The invention has the advantage that the potential differences between the plates of the different pairs may be adjusted independently and as desired so that the required correct action can be obtained without having to locate and dimension the different pairs, and their inter-plate spacings with precision. It is indeed possible (and preferred) to make all the pairs to the same nominal design and it will not matter, if, by reason of manufacturing or other errors, the nominal design is not accurately complied with, for departures from the nominal design can be readily dealt with by independently adjusting the applied potentials.

In the simplest embodiments of the invention one plate of each pair is connected to one plate of each other pair. This interconnection may be made, if desired, inside the tube envelope and brought out of the envelope by a single external connection. The other plates are connected to separate connections which are brought outside the tube envelope. In use they are connected to points one or more of which may be adjustable, along a potentiometer across which modulating potential is applied. The potentiometer is conveniently mounted on the outside of the tube envelope and arranged for sealing after factory adjustment in manufacture or alternatively the adjustments may be arranged for the user to make during operation of the tube to permit ready adjustment to take care of variations during working.

The invention is illustrated in the accompanying drawing which shows schematically and sufliciently for present purposes part of a cathode ray tube employing the invention.

In the drawing part of the neck of a cathode ray tube is shown at 1. The normal i.e the undeflected direction of the electron beam from the electron gun is, as indicated by chain line, down the common axis of the tube and of the neck. The beam passes first between deflection plates 3 and 4 and then between a similar pair of compensating plates 5 and 6 on its way to an apertured electrode 7 which is mounted transverse to the undeflected beam path and has a hole 8 centered on the axis. In the absence of applied deflection the beam passes unobstructed through the hole 8.

In order to modulate the beam deflection potential is applied between plates 3 and 4. This deflects the beam causing it to be intercepted by the electrode 7 to an extent depending on the amount of deflection. In the limit the beam is cut off entirely, being so deflected that the whole of its cross section is intercepted by the electrode 7.

Between the plates 3, 4 and the electrode 7 is a pair of compensating plates 5 and 6 for compensating for undesired shifting, as a result of modulation, of the final point of impact of the beam. The plates 3 and 6 on opposite sides of the beam are connected together inside the tube envelope and brought out by connection 10 to one end (shown earthed) of a potentiometer 9, mounted outside the envelope. The plate 5 is brought out by its own connection 11 to the other end of potentiometer 9 and the plate 4 is brought out, again by its own connection 12, to an adjustable tap on potentiometer 9 Where (the common case) the tube has an anode (not shown) intended to operate at earth potential, the electrodes 3 and 6 may be internally connected to the anode connection lead. Modulatory potentials, represented, as a practical example, as a negative going pulse of, say, 30 volts, is applied across the resistance 9.

I claim:

1. A deflection modulated beam tube having a cathode and comprising an apertured electrode across which an electron beam is deflected, in order to achieve modulation, a pair of deflecting plates for deflecting said beam across said electrode, a pair of compensating plates one plate of which is connected to one plate of the pair of deflecting plates for compensating for undesired shifting of the point of final beam impact, and separate connections extending outside the tube from at least one plate of each pair of plates, each of said connections being connected to separate points along a potentiometer on the outside of said tube whereby the potentials applied to the said one plate of each pair are adjustable inter se.

2. A tube as claimed in claim 1 wherein all the plates are alike and all the pairs have the same inter-plate separation.

3. A tube as claimed in claim 2 wherein the compensating plates include a pair on the cathode side of said apertured electrode.

4. A tube as claimed in claim 2 wherein the compensating plates include a pair on the side of said apertured electrode remote from the cathode.

5. A tube as claimed in claim 4 wherein one plate of each pair is connected to One plate of each other pair.

References Cited UNITED STATES PATENTS 2,302,786 11/1942 McGee et a1 31387 X 2,361,998 11/ 1944 Fleming-Williams 3153() 3,151,272 9/1964 Crowell 3 l530 3,256,460 6/1966 Wendt 3153O RICHARD A, FARLEY, Primary Examiner.

MALCOLM F. HUBLER, Assistant Examiner.

US. Cl. X.R. 313-87 

